Thin film transistors (TFT) used for image displays in the prior art have been formed using, as a device material, polycrystalline silicon formed by a melting recrystallization method such as excimer laser annealing from amorphous silicon or microcrystalline silicon formed by a plasma CVD process on an insulative substrate such as glass or quartz as a base material.
The semiconductor device (TFT) and the manufacturing method thereof in the prior art are to be described with reference to FIG. 17(a) to (d). As shown in FIG. 17(a), an amorphous silicon thin film 202 is deposited on one surface of a glass substrate 201.
Then, as shown in FIG. 17(b), when the surface of the amorphous silicon thin film 202 is scanned by a linear excimer laser beam 204 in the direction of an arrow 203, the amorphous silicon thin film 202 is heated by the excimer laser beam 204 and changed from an amorphous structure into a polycrystalline structure. When the entire surface of the amorphous silicon film 202 is heated by the excimer laser beam 204 under scanning, a polycrystalline silicon thin film 205 is formed as shown in FIG. 17(c). In FIG. 17(c), the polycrystalline silicon thin film 205 is made of silicon crystal grains and a grain boundary 206 is formed between the crystal grains.
The process described above is referred to as a laser heating process. This is adopted when a polycrystalline silicon thin film at high quality is prepared on a substrate comprising a low melting material such as glass. They are described in details, for example, in “1996 Society for Information Display International Symposium Digest of Technical Papers, pp 17-29” and “IEEE Transactions on Electron Devices, vol. 43, No. 9, 1996, pp 1454 to 1458” and the like.
FIG. 17(d) shows a transistor (TFT) formed by using the polycrystalline silicon thin film in FIG. 17(c).
A gate insulation film 208 such as a silicon oxide film is disposed on the polycrystalline silicon thin film 205. Further, a source impurity implantation region 207 and a drain impurity implantation region 209 are formed on the polycrystalline silicon thin film 205. A thin film transistor is formed by disposing a gate electrode on the source-impurity implantation regions 207 and 209 and the gate insulation film 208.
FIG. 18 shows the dependence of the size of the silicon crystal grain and the roughness of the polycrystalline silicon thin film on the irradiation laser energy in the prior art (dependence 301 of the size of the crystal grain on the laser energy density). Silicon is not crystallized at the energy with a laser energy density of 200 mJ/cm2 or less but crystallization initiates when it exceeds 200 mJ/cm2 and the size of the crystal grain increases along with an increase in the laser energy density.
However, when the laser energy density exceeds 250 mJ/cm2, the silicon crystal grain becomes smaller. Since a polycrystalline silicon thin film transistor having favorable characteristics may be manufactured by increasing the size of the silicon crystal grains, the energy density of the laser is set to 250 mJ/cm2.
The value of the laser energy density in the prior art may sometimes vary since it depends on the nature of the amorphous silicon film (for example, growing method, film thickness). They are described in details, for example, in “Applied Physics Letters, vol. 63, No. 14, 1993, pp 1969-1971”.
Further, for increasing the grain diameter of the crystal grains, laser irradiation may preferably be conducted by heating the substrate at 400° C. This is because the solidification velocity is lowered by heating the substrate and the grain diameter increases up to about 500 nm. Further, since a temperature gradient is caused at the end of the laser beam, the size of the crystal grains varies remarkably. In order to prevent this, laser may be preferably irradiated under overlapping. They are reported in “Proceedings of The Institute of Electronics, Information and Communication Engineers C-II Vol. J76-C-II, 1993, pp 241-248”.
Further, for making the size of the crystal grains uniform, first laser irradiation is applied at first at a low energy density and, subsequently, a second laser irradiation is applied at a high energy density required for crystallization. Such a two-step laser irradiation is applied for forming crystal seeds by the first laser irradiation and crystallization of them by the second laser irradiation. In this case, while the uniformess is improved, the crystal grain diameter is reduced. This is reported in “Proceedings of 42th Laser Materials Processing Conference, 1997, pp. 121-130”.